The present invention relates to a circuit arrangement for protecting an electrical component from undesirably high values of electrical potential, and more particularly a safeguard feature in such a circuit arrangement which prevents continued supply of voltage to the electrical component in the absence of the proper protective function of the circuit arrangement.
Circuit arrangements of this type are generally employed when an electrical device is operated by a power supply in which an unacceptably high output voltage may occasionally occur. A common application for such arrangement, for example, is in conjunction with a typical power supply present in a motor vehicle comprising a three phase generator with a regulator and battery. In such systems, voltage peaks produced by the three phase generator are not reliably suppressed by the regulator or the battery in all instances. Consequently, under certain conditions, undesirable voltage peaks output from the power supply may reach the sensitive electrical devices supplied thereby. In such cases, the electrical devices may be damaged by the surge in voltage in the absence of further protective measures.
A known circuit arrangement for protecting an electrical device from excessively high voltage peaks, and which is often incorporated in the device itself, is connected in parallel with the component to be protected, and is located between the source of the electrical potential, i.e. generally the power supply, and the component. Specialized circuit elements, referred to as suppressor diodes and having the unique characteristic of selectively conducting when exposed to a threshold voltage potential level above normal operating voltage, are frequently used as protective elements in such circuit arrangements. When a voltage in excess of the threshold triggering value of the suppressor diode is reached, i.e. the result of an unacceptably high voltage peak, a connection is established across the terminals thereof Voltage is thereby short-circuited within the protective circuit portion of the electrical device, and does not therefore reach the component to be protected. A fuse is also provided in or before the electrical device which opens to avoid damage resulting from the resultant short-circuit.
Protective response of the known circuit arrangement to an inadmissible voltage peak is accompanied by a simultaneous intense warming of the protective element. Since the protective element is generally mounted on a electrical printed circuit and conductively connected to same by means of soldered connections, heat generated by the protective element during a shorting thereof is transferred to the solder. As the temperature of the solder is increased beyond the melting point, the soldered connections are liquified, and may flow off. In some cases the heat generated by the protective element is intense enough to cause the solder holding it in place to evaporate. In either event, mechanical attachment, and thereby secure electrical connection of the protective element in the circuit, is no longer insured.
Even when the solder does not flow off or evaporate, a secure connection of the protective element is not always insured once the solder has liquefied. For example, in applications in which mechanical vibrations act upon the protective element or the electrical device, the danger exists that the protective element may be dislodged from the printed circuit before the protective effect occurs or before the fuse is destroyed, insofar as the protective element is mechanically attached exclusively by the soldered connection.
The above-mentioned danger can be reduced by mechanical attachment of the protective element by means supplemental to soldering, for example by use of adhesive or by bending of the element leads. The latter measure would not however be a feasible option in connection with surface-mounted devices (SMD). It is yet another disadvantage of such attachment methods that additional effort, and thereby increased costs, occur in the manufacture of the electrical device as a result thereof.
It is therefore an object of the invention to provide a circuit arrangement for protecting an electrical component from undesirable values of electrical potential, in which reliable protection of the electrical element from such voltage surges is made possible in a simple manner, using conventional protection elements.